An oscillation circuit is a typical temperature sensitive circuit whose normal operation is impaired by the effect of the ambient temperature. This is because the frequency characteristics of the resonator of the oscillation circuit have temperature dependence such as a cubic curve. Accordingly, it is necessary for the temperature sensitive circuit to correct its operation in response to the temperature. A conventional temperature correcting apparatus used for this purpose generally detects the ambient temperature with a temperature sensor, generates a correcting voltage in response to the detected ambient temperature, and carries out correcting control by supplying the correcting voltage to the apparatus to be corrected such as an oscillation circuit. However, it is necessary for the apparatus to be corrected, which requires highly accurate temperature correction over a wide temperature range, to subdivide the temperature range to make correction of the individual subdivisions, and to consider the detection error of the temperature sensor together with the reduction in the effect on the variations in the detection characteristics of the sensor.
A conventional temperature correcting apparatus meeting the foregoing requirements is disclosed in Japanese patent application laid-open No. 2001-68996. It converts a plurality of temperature detection values to addresses, corrects the temperature characteristics of the temperature sensor in response to the individual addresses, and stores the correcting addresses in a storage in advance. In addition, it stores correcting data, which are determined by considering the correcting voltages for correcting the temperature characteristics of the apparatus to be corrected, in the storage in advance in correspondence to the individual correcting addresses. Then, it detects the ambient temperature with a temperature sensor, carries out the A/D conversion of the detected temperature value to an address, and reads the correcting address corresponding to the converted address. Subsequently, it reads the correcting data according to the correcting address read out, and D/A converts the correcting data to produce a correcting voltage to be fed to the apparatus to be corrected. To achieve highly accurate correction by the technique, however, an increasing amount of the D/A data is required. On the other hand, Japanese patent application laid-open No. 2001-28514 discloses a technique for reducing memory capacity by storing differential data with respect to a specified temperature into a memory instead of storing all the D/A data.
With the foregoing configuration, the conventional temperature correcting apparatus must consider the quantization error of the D/A converter. This is because when the D/A converter generates the voltage waveform for correcting a given temperature, the data is input at constant sampling intervals. Thus, to reduce the error of the D/A converter, which consists of the quantization error always accompanying the quantization, a higher bit D/A converter is required. Thus using the higher bit D/A converter results in an increase in the amount of the D/A data and in the cost. On the other hand, Japanese patent application laid-open No. 2001-28514, which calculates the D/A data by the correcting calculation, has a problem of bringing about quantization error at every voltage output carried out by the D/A conversion at constant sampling intervals.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a temperature correcting apparatus that carries out highly accurate correction by curbing the occurrence frequency of the quantization error of the D/A converter even by using a low bit D/A converter, and that implements low cost temperature correcting processing, and a voltage-controlled oscillation apparatus using the temperature correcting apparatus.